The present invention relates to a protecting member for a gasket installed in an internal combustion engine, which can prevent creep relaxation of the gasket caused by repeated changes or alterations of forces applied to the gasket through bolts for connecting the engine parts.
As shown in FIG. 1, in the conventional internal combustion engine, when a cylinder head CH is attached to a cylinder block CB, a gasket G is installed between the cylinder head CH and the cylinder block CB, and is securely held therebetween by bolts BO. In case the engine is actuated, when a fuel is combusted in a cylinder bore Hc, large force F is applied to the cylinder head CH, and when the combusted gas in the cylinder bore Hc is exhausted, the pressure applied to the cylinder head CH is released.
Thus, when the force F due to combustion of the fuel in the cylinder bore Hc is applied to the cylinder head CH, tension T1 is applied to the bolts BO through the cylinder head CH, so that tightening pressure applied to the gasket G is temporarily reduced. On the other hand, in the exhaust cycle of the engine, since the tension T1 is suddenly released, tension applied to the bolts BO returns to tension T2 equal to the tightening pressure to the bolts BO, by which the tightening pressure to the gasket G increases again.
As explained above, when the engine is actuated, tension applied to the bolts BO changes repeatedly, by which tightening pressure applied to the gasket G changes as well. The changes of the tightening pressure or tension to the bolts BO cause creep relaxation of the gasket G. Especially, in case the gasket includes beads for sealing around holes, the beads may be damaged by the changes of the tightening pressure.
In case the cylinder head CH is made of an aluminum alloy, thermal expansion and conductive rates are different from those of the bolts BO. Thus, when the engine is started, the cylinder head becomes hot and expands at first, and then the bolts BO become hot. Accordingly, after the cylinder head CH expands and before the bolts BO expand by heat, tightening pressure to the gasket G becomes high, which exceeds the initial tightening pressure applied to the bolts BO. Also, when the engine is stopped and is cooled, the tightening pressure decreases.
Therefore, in a cold area where the difference between the driving temperature and the ambient temperature is great, the change of the tightening pressure by the bolts BO is large, and causes creep relaxation of the gasket.
The kind and degree of the creep relaxation of the gasket or bead depend on the quality of the material that the gasket is made. In case the gasket is formed of a spring steel plate, creep relaxation of the bead does not occur so easily. However, in case the changes of the pressure applied to the gasket are large, the spring ability or resiliency of the bead decreases gradually, and the bead does not function as installed at first to thereby cause leakage of fluid through the bead.
Accordingly, one object of the invention is to provide a protecting member for a gasket, which can effectively prevent creep relaxation of the gasket.
Another object of the invention is to provide a protecting member for a gasket as stated above, which can reduce changes of tension applied to bolts regardless the condition of the engine.
A further object of the invention is to provide a protecting member for a gasket as stated above, which can be formed as a part of the gasket.
A still further object of the invention is to provide a protecting member for a gasket as stated above, which can be manufactured easily and economically.
Further objects and advantages of the invention will be apparent from the following description of the invention.